1. Field of the Invention
The present invention relates to a method of polishing a large part by projecting and colliding abrasives to a surface of the large part, and an abrasive for use in the method.
2. Description of the Related Art
In large parts such as a steam turbine, moving and stationary vanes, a turbine rotor, or parts of fluid passage (a steam valve, steam tube, crossover tube, turbine inlet, outlet, or nozzle box inside) of the steam turbine, the surface roughness of the large parts has a key factor on the turbine performance, and it is required to improve the surface state by polishing.
As a representative example of the large parts, a schematic configuration of a general steam turbine will be explained by referring to FIGS. 6 and 7.
FIG. 7 is a schematic sectional view showing an entire steam turbine. A turbine rotor 1 has about a hundred moving vanes set in the peripheral direction to form vane rows, and the vane rows are disposed at mutual intervals in different lengths of moving vanes 1a in the axial direction, depending on the pressure and temperature of steam passing therethrough.
A turbine casing 2 comprises nozzle diaphragms 3 shown in FIG. 6 disposed between the vane rows. The nozzle diaphragm 3 is formed of a nozzle diaphragm inner ring 4 and a nozzle diaphragm outer ring 5, and a stationary vane 6 is held between the rings.
By installing the turbine casing 2, stationary vanes 6 of the nozzle diaphragm 3 are disposed between the vane rows in the axial direction of the turbine rotor 1.
As a result, in the axial direction of the turbine rotor 1, the moving vanes 1a and stationary vanes 6 are disposed alternately, and a turbine stage is formed by the combination of a set of the moving vane and stationary vane.
By arranging such turbine stages in several stages, a high pressure turbine 7, a medium pressure turbine 8, and a low pressure turbine 9 are formed.
Steam flow of the steam turbine will be explained below.
In FIG. 7, steam of high temperature and high pressure introduced from a boiler (not shown) is first sent into the high pressure turbine, and heat energy is transformed into mechanical rotary energy by each turbine stage, so that the high pressure turbine 7 is put into rotation.
The steam having worked in the high pressure turbine 7 is sent into a reheater in the boiler again, regenerated into steam of high temperature and high pressure again, and sent into the medium pressure turbine.
The steam having worked and rotated the medium pressure turbine 8 is directly exhausted into a crossover tube 10, and flows in the inside of the crossover tube 10 and enters the low pressure turbine 9.
The steam having entered the low pressure turbine 9 similarly works by rotating the low pressure turbine 9, and is then exhausted into a condenser 11, at which the steam is condensed into water. The condensed water is sent back into the boiler to be steam, and is introduced into the turbine. In this manner, the circulation is repeated.
In the steam turbine having such a configuration, in order to enhance the performance, it is required to polish the surface of turbine parts so as to eliminate the roughness, thereby lowering the passage resistance when the steam flows.
FIG. 8 shows the turbine stage efficiency when the surface roughness is improved, supposing the efficiency in the turbine stage at surface roughness of Ry 6.3 in the current design specification to be 100.
As known from the diagram, by finishing more smoothly in the steam passing parts in the turbine stage composed of moving vanes and stationary vanes, the efficiency is improved by about 3.5% from the current level.
At the present, technical developments are attempted by various methods for improvement of efficiency of a steam turbine, and further for improvement of power generation efficiency, and such a method not requiring major design change or modification of equipment is widely noticed, and has been already applied in actual machines.
However, for example, since one moving vane is longer than 1 meter and the shape is extremely complicated, narrow and complicated parts must be polished, and it is hard to polish mechanically or automatically.
In the conventional work of polishing turbine parts, compressed air or a power rotary tool such as a so-called grinder has been used, or liquid, paper, cloth, chemical fiber or the like having polishing effect has been used in manual polishing work known as buffing.
Such a method of polishing turbine parts requires much time and cost.
Recent methods of polishing a material to be polished include a sand blasting method of polishing the surface of parts to be polished by projecting ceramic projection materials by the compressed air.
In the sand blasting method, the surface can be cleaned, the film can be removed in the entire region of projection range, and the polishing performance is excellent. However, the surface thickness may be reduced excessively, and the surface roughness may be worsened, so that there are environmental problems such as scattering of dust particles. In turbine parts, in particular, it is hard to obtain the surface roughness of Ry 6.3 or Ra 1.0 or less in the finished state.
On the other hand, the steam turbine is inspected periodically, and parts through which high temperature steam passes, such as the moving vanes 1a and stationary vanes 6 in the inside, are inspected.
The steam contains traces of impurities called oxide scales, and the oxide scales are collected on the moving vanes 1a and stationary vanes 6 during operation for a long period of time. Therefore, oxide films often deposit on the surface.
Such oxide scales and oxide films are known to lower the precision of nondestructive inspection extremely at the time of periodic inspection. In nondestructive inspection, liquid penetrates from the surface into the inner parts, or X-ray or ultrasonic wave is emitted to inspect the inside state by reflected waves from the inside. Therefore, when the roughness of the surface as the access of information from inside is poor, the information from the inside is disturbed, and the precision of inspection is lowered.
Accordingly, at the time of periodic inspection, such oxide scales and oxide films are necessarily removed, and the surface roughness is minimized by manual work. It hence takes much time and labor, and because of manual work, the surface roughness degree varies, and the precision of inspection is not always excellent.
Besides, the oxide scales and others depositing on the moving vanes 1a and stationary vanes 6 cause to change the vane sectional shape at the time of designing, so that the performance of the steam turbine itself may be also lowered. Accordingly, at the time of periodic inspection, such oxide scales or the like are scraped off. Since the rear edges of the vanes are very thin in particular, deformation may be increased in the sand blasting method.
In a known method of grinding a work surface, abrasive grains having grinding powder adhered to a carrier made of elastic and porous vegetable fibers by using fat or sugar contained in vegetable fibers as an adhesive are injected in multiplicity obliquely from the work surface by mixing with a grinding fluid, and the abrasive grains slide on the work surface while elastically deforming the carrier, so that the work surface is finished by the grinding powder (Jpn. Patent No. 2957492, for example).